In recent years, the control of semiconductor processes has evolved to include an approach referred to as run-to-run (RtR) control. RtR control is a type of supervisory-level control that uses in-line measurements to adjust the recipe used on a process tool. The recipe adjustments are typically made on a lot-to-lot, wafer-to-wafer, or batch-to-batch basis to compensate for drifting tool qualities or changes in incoming wafer conditions. The in-line measurements are made after the process begins being controlled in the case of feedback control, or before the process in the case of feedforward control.
While RtR controllers have been applied or proposed for various fabrication processes, one important issue that affects deployment of RtR control has largely been ignored. In many state-of-the-art fabs, costly fab tools are required to run more than one process for throughput and flexibility reasons. Furthermore, such fabs often produce more than one type of chip. Multi-process and multi-product considerations can lead to difficulties when designing and deploying a RtR controller.
Many conventional RtR controllers have been concerned with the control of a single product type going through one process. However, most applications in an operable state-of-the-art fab will be faced with many products and product types. For example, along with the fab's principal product, test chips and secondary products are typically produced as well. Further, in order to improve the flexibility of the fab and reduce tool costs, multiple processes may be run on the same tool. A tool may therefore be required to manage multiple products using one process, multiple processes for a given product, and/or multiple processes and multiple products. These potentially diverse circumstances can lead to complications in RtR control practice.
An example of a single-process, multiple product fab tool might be a poly gate etch tool. Poly gate etch involves the smallest linewidth dimensions on the chip, and as such usually presents a difficult control problem. Since poly gate etch is a process used in the manufacture of most semiconductor chips, all of the products manufactured in a fab will have to go through the process. However, as it is such a critical process and is often difficult to control, many fabs dedicate equipment to this process. While this may involve only a single process, RtR control for such a tool still requires consideration for multiple products.
On the other hand, some fabs are dedicated to only one product. This is common in the manufacture of memory chips, where high volumes of a single type of chip are commonplace. In these fabs, the issue of controlling multiple products through a process tool may not surface. However, in smaller fabs where tools cannot be dedicated to a particular process, a RtR control system must be able to comprehend these multiple processes and be able to effectively control them.
The most complex case for a RtR controller is that of multiple processes and multiple products running on the same process tool. An example of this is Chemical-Mechanical Polish (CMP) of interlayer dielectric (ILD) layers. Multiple ILD layers are typical in microprocessors, and the use of CMP to planarize the ILD layers is becoming more and more common. Generally, CMP utilizes an abrasive slurry disbursed in an alkaline or acidic solution to planarize the surface of the wafer through a combination of mechanical and chemical action. A typical chemical mechanical polishing tool includes a rotatable circular platen or table on which a polishing pad is mounted and a polishing device is positioned above the pad. The polishing device includes one or more rotating carrier heads to which wafers can be secured typically through the use of vacuum pressure. In use, the platen is rotated and an abrasive slurry is disbursed onto the polishing pad. Once the slurry has been applied to the polishing pad, a downforce is applied to each rotating carrier head to press its wafer against the polishing pad. As the wafer is pressed against the polishing pad, the surface of the wafer is mechanically and chemically polished. These polish operations are often similar, which allows for CMP tools to switch between them. However, they are not identical, and changes in wafer topography and target thickness result in differences in the processes.
Accordingly, there is a need for a control technique capable of facilitating multi-product and multi-process runs in semiconductor fabrication systems, and for reducing tool costs and improving the flexibility of the fab. The present invention provides a solution to the aforementioned and other shortcomings of the prior art, and provides advantages over existing run-to-run control methodologies.